1. Field of the Invention
The present invention relates to a method for making a heat-sensitive, negative-working lithographic printing plate precursor.
2. Description of the Related Art
Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to the image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e., ink-accepting, water-repelling) areas and hydrophilic (or oleophobic, i.e., water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas, and during driographic printing only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called a plate precursor. In addition to the well-known photosensitive, so-called pre-sensitized plates, which are suitable for UV contact exposure through a film mask, heat-sensitive printing plate precursors have also become very popular in the late 1990s. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e., without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by crosslinking of a polymer, heat-induced solubilization, or by particle coagulation of a thermoplastic polymer latex.
Although some of these thermal processes enable plate making without wet processing, the most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The coating typically includes an oleophilic binder, e.g., a phenolic resin, of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working), by the image-wise exposure. During processing, the solubility differential leads to the removal of the non-image (non-printing) areas of the coating thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support. Typical examples of such plates are described in, e.g., European Patent Nos. 625728, 823327, 825927, 864420, 894622, and 901902. Negative working embodiments of such thermal materials often require a pre-heat step between exposure and development as described in, e.g., European Patent No. 625728.
Negative working plate precursors which do not require a pre-heat step may contain an image-recording layer that works by heat-induced particle coalescence of a thermoplastic polymer latex, as described in, e.g., European Patent Nos. 770494, 770495, 770496, and 770497. These patents disclose a method for making a lithographic printing plate including the steps of (1) image-wise exposing an imaging element including hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and a compound capable of converting light into heat, and (2) developing the image-wise exposed element by applying fountain solution and/or ink.
Another plate that works by latex coalescence is described in European Patent No. 800928, which discloses a heat-sensitive imaging element including on a hydrophilic support an image-recording layer including an infrared absorbing compound and hydrophobic thermoplastic particles dispersed in an alkali soluble or swellable resin which contains phenolic hydroxyl groups.
A similar plate is described in U.S. Pat. No. 6,427,595 which discloses a heat-sensitive imaging element for making lithographic printing plates including, on a hydrophilic surface of a lithographic base, an image-recording layer including a compound capable of converting light into heat and hydrophobic thermoplastic polymer particles, which have a specific particle size and polydispersity, dispersed in a hydrophilic binder.
European Patent Nos. 514145 and 599510 disclose a method for forming images by direct exposure of a radiation sensitive plate including a coating including core-shell particles having a water insoluble heat softenable core compound and a shell compound which is soluble or swellable in an aqueous alkaline medium. Image-wise exposure with infrared light causes the particles to coalesce, at least partially, to form an image, and the non-coalesced particles are then selectively removed by an aqueous alkaline developer. Afterwards, a baking step is performed.
European Patent No. 950517 discloses a lithographic printing plate precursor consisting of a lithographic base with a hydrophilic surface and an IR-sensitive top layer including a polymer soluble in an aqueous alkaline solution and a polysiloxane surfactant.
European Patent No. 1,462,252 discloses a positive-working heat-sensitive printing plate precursor including, on a support having a hydrophilic surface, a coating including a cross-linked polysiloxane spacer particle with a particle size between 1 μm and 15 μm, an infrared absorbing agent, an oleophilic resin soluble in an aqueous alkaline solution, and a developer resistance means.
European Patent No. 1,243,413 discloses a method for making a negative-working heat-sensitive lithographic printing plate precursor including the steps of (i) applying on a lithographic base having a hydrophilic surface an aqueous dispersion including hydrophobic thermoplastic particles and particles of a polymer B which have a softening point lower than the glass transition temperature of the hydrophobic thermoplastic particles, and (ii) heating the image-recording layer at a temperature which is higher than the softening point of polymer B and lower than the glass temperature of the hydrophobic thermoplastic particles.
U.S. Pat. No. 5,948,591 discloses a heat sensitive element for making a lithographic printing plate including, on a base having a hydrophilic surface, an image-recording layer including an infrared absorbing agent, hydrophobic thermoplastic particles, and a copolymer containing acetal groups and hydroxyl groups which are at least partially reacted with a compound with at least two carboxyl groups.
European Patent No. 832,739 discloses a heat-sensitive element including, on a support having an ink-accepting surface, an image-forming layer containing hydrophobic thermoplastic polymer particles and a compound capable of converting light into heat, and a cured ink-repellent surface layer.
U.S. Pat. No. 6,737,220 discloses a printing plate precursor including a support onto which a coating liquid containing thermoplastic particles and a water-soluble material such as a saccharide is applied, wherein the coating liquid may include a water-soluble silicon or fluorine containing surfactant to improve its coatability.
European Patent No. 849090 discloses an imaging element for making a lithographic printing plate including on a flexible support (i) an ink-repellent layer including a cross-linked hydrophilic binder, (ii) a thermo-sensitive layer including hydrophobic thermoplastic particles dispersed in a hydrophilic binder, and (iii) an outermost layer on top of the layers including a solid or liquid lubricant in a hydrophilic binder.
European Patent No. 1,428,676 discloses a printing material including on an aluminum support an image forming layer including thermoplastic particles and a light-to-heat conversion dye, wherein the imaging forming layer may further include a water-soluble resin and/or a water-soluble silicon or fluorine atom-containing surfactant.
Printing plate precursors are susceptible to damage caused by mechanical forces applied to the surface of the coating during automatic transport, mechanical handling, and/or manual handling. The risk of damage occurs especially before and after the imaging step prior to the processing step. In a typical platesetter, the plate precursors are conveyed by a mechanical device, e.g., rollers or suction cups/devices which are applied to the surface of the precursors and thereby may cause damage to the coating. Rollers may, for example, cause latex particles to partially coalesce thereby forming ink-accepting areas at non-image areas, while suction cups may destroy the coating resulting in disturbed image areas. Furthermore, after coating and drying, the thermal printing plates are stacked and are then, by means of specified packaging equipment, cut and packed in boxes. During cutting and packing of the printing plate precursors as well as during transport of the packed printing plate precursors, the plates can move relative to each other whereby the heat-sensitive coating is rubbed which also may result in surface damage. Moreover, the manual handling of the printing plate precursors may result in so-called fingerprints which lead to a reduced printing quality.
Thus, the major problems associated with the prior art plate materials that work by latex coalescence is that they are easily damaged by automatic plate handling systems and/or by mechanical and manual contact, wherein this damage results in a reduced printing quality due to a destruction of the surface of the coating of the printing plate precursor or to a pressure-induced coalescence of the latex particles in the image recording layer.